In recent years, blue high-output semiconductor lasers have been developed, and high-density recording technology for optical discs that uses this short-wavelength laser light has been developed and adopted for practical use. In principal, blue semiconductor lasers have a larger forward voltage than do conventional red semiconductor lasers, and thus for a given forward voltage consume more power and produce more heat. For this reason, when a blue semiconductor laser is mounted to an optical head, there is a large rise in temperature due to the generation of heat, and thus the reduction of power in other blocks in the optical head and improvements in heat radiation have become important issues.
Conventionally, one method that has been used to reduce power consumption by the semiconductor laser driving circuit that is used in an optical disc device is the method of, in MD devices for music playback only, for example, to directly connect the semiconductor laser and the DC-DC converter to one another and then apply feedback based on the monitor results of the laser output of the semiconductor laser. With this method, the series voltage drop between the semiconductor laser and the DC-DC converter is a minimum, and moreover, in the DC-DC converter there is little voltage drop loss like in voltage regulators, and thus this method is ideal from the perspective of reducing power consumption. However, one issue with this method is that the response speed when switching from playback to record is not sufficient, and thus as it is, the method is not suited for recording devices.
The semiconductor laser driving circuit of Patent Document 1 is an example where the response speed is improved. Patent Document 1 discloses an “anode drive” type semiconductor laser driving circuit that performs control of the semiconductor laser by controlling the current that flows to the anode of the semiconductor laser, whose cathode is grounded. The forward voltage of the semiconductor laser (hereinafter, “Vop”; it should be noted that here this is equal to the anode voltage) varies by semiconductor laser and changes depending on the value of the forward current (If) of the semiconductor laser, and thus when Vop is equal to or less than a predetermined value, the voltage that is applied to the semiconductor laser driving circuit may be lowered by lowering the output voltage of the DC-DC converter, thereby suppressing unnecessary power consumption. In other words, if the Vop is equal to or less than a predetermined value, a semiconductor laser driving circuit that is low power and that switches between the record power and the playback power at high speed by switching the output voltage of the DC-DC converter to a low voltage that has been determined in advance is achieved.
Patent Document 2 discloses an anode drive-type semiconductor laser driving circuit whose aim is to reduce power consumption. In the semiconductor laser driving circuit of Patent Document 2, to accurately measure the circuit voltage, the output voltage of the DC-DC converter and the Vop are each measured through a level shift circuit and a peak hold circuit, and the difference between these two is calculated to measure the voltage that is applied to the semiconductor laser driving circuit, and the output voltage of the DC-DC converter is controlled based on this result.
Patent Document 3 discloses an anode drive-type semiconductor laser driving circuit. In the semiconductor laser driving circuit of Patent Document 3, the Vop of a blue semiconductor laser is measured through a peak hold circuit or an integrating circuit, for example, and the output voltage of the DC-DC converter is controlled so that a voltage that is equal to the voltage obtained by adding the voltage necessary for the semiconductor laser driving circuit to the Vop is output.
Patent Document 1: JP 2000-244052A
Patent Document 2: JP 2002-158395A
Patent Document 3: JP 2002-260266A